Camptix: add notification rule to dismiss addresses that are following via jetpack

Description

If organizer sends a notification to attendees AND publishes blog post with exact (or very close) same information, some amount of attendees will get the information twice because they are also subscribed to updates from blog.

We can't get the email list from Jetpack at the time being, but maybe there's a chance to talk Jetpack's team if wc.org could have "private" way of getting those emails? After that we can make camptix tweak to prevent duplicates id organizer want's so.

As a frequent WordCamp attendee myself, I think getting information twice is always preferable over not getting it at all. Has this been a problem that has been brought up repeatedly, or rather something you assume could be a nuisance to some?

I don't think this is something we'll likely prioritize in the near or mid term. Coordinating something like this w/ would likely be a lot of work, and it's not clear to me that this is a significant pain point for a significant number of people.

We can always revisit this if/when new information or opportunities emerge, but I think for now it'd be better to focus our limited time and energy on things that are high-impact or low-effort.